Polyester resins have found widespread use in varied applications. Polyester resins, such as polyethylene terephthalate and polyethylene naphthalate, are used in films, including photographic film and magnetic tape, in fibers, and in food containers such as beverage containers. Various methods have been disclosed for the depolymerization of such resins into their component monomers, such as ethylene glycol, terephthalic acid, naphthalic acid, or derivatives thereof, so that they could be reused.
Naujokas et al. U.S. Pat. No. 5,051,528 describes a process of recovering ethylene glycol and dimethyl terephthalate from polyethylene terephthalate scrap resins by dissolving the polyester resin in oligomers of the same monomers as present in the polyester, passing super-heated methanol through the solution and recovering ethylene glycol and dimethyl terephthalate.
Gamble et al. U.S. Pat. No. 5,298,530, issued Mar. 29, 1994 describes improvements in the process of the '528 patent in which the scrap resin is combined with reactor melt in a dissolver, before the dissolver melt is transferred to the reactor for contact with super-heated methanol. In the reactor, polymers and oligomers are futher depolymerized into the component glycol and ester monomers, which are then recovered.
The processes described in these patents have numerous advantages. These include low cost, high efficency and the ability to be used with a variety of forms of polyester of varying degrees of cleanliness and purity. Thus, suitable feedstock to the dissolver can be magnetic tape, polyester food containers, such as soda bottles, and photographic film, which may contain residual layers containing trace amounts of silver halide, magnetic iron oxide and other compounds found in photographic films, such as sulfur compounds. In addition, the processes are sufficently adaptable that they can be used with feed stock other than polymer. For example, they could be used to recover components from polyester oligomer, or to purify monomer, such as dimethylterephthalate, that has become contaminated during or after its manufacture.
However, a problem with the process is that under some conditions p-dioxane (hereinafter, dioxane) is formed. Since dioxane is a potentially harmful material, it is highly desirable to avoid is formation.